Apparatus for polishing thin, flat semi-conductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is "planarized" or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 10 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, the appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing a metal oxide may be formed and removed repeatedly.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. Polishing heads of the type described above used in the CMP process are shown in U.S. Pat. Nos. 4, 141180 to Gill, Jr., et al.; 5, 205, 082 to Shendon et al; and, 5, 643,061 to Jackson, et al. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by non-uniform pressure on a wafer. The problem is reduced somewhat through the use of a retaining ring which engages the polishing pad, as shown in the Shendon et al patent.
Referring now to FIG. 1C, wherein an improved CMP head, sometimes referred to as a Titan.RTM. head which differs from conventional CMP heads in two major respects is shown. First, the Titan.RTM. head employs a compliant wafer carrier and second, it utilizes a mechanical linkage (not shown) to constrain tilting of the head, thereby maintaining planarity relative to a polishing pad 12, which in turn allows the head to achieve more uniform flatness of the wafer during polishing. The wafer 10 has one entire face thereof engaged by a flexible membrane 16, which biases the opposite face of the wafer 10 into face-to-face engagement with the polishing pad 12. The polishing head and/or pad 12 are moved relative to each other, in a motion to effect polishing of the wafer 10. The polishing head includes an outer retaining ring 14 surrounding the membrane 16, which also engages the polishing pad 12 and functions to hold the head in a steady, desired position during the polishing process. As shown in FIG. 1C, both the retaining ring 14 and the membrane 16 are urged downwardly toward the polishing pad 12 by a linear force indicated by the numeral 18 which is effected through a pneumatic system.
In the improved CMP head 20 shown in FIG. 1C, large variations in the removal rate, or polishing rate, across the whole wafer area are frequently observed. A thickness variation across the wafer is therefore produced as a mean cause for wafer non-uniformity. The improved CMP head design, even though utilizing a pneumatic system to force a wafer surface onto a polishing pad, the pneumatic system cannot selectively apply different pressure at different locations on the surface of the wafer. For instance, as shown in FIG. 1D, a profilometer data obtained on an 8-inch wafer is shown. The thickness difference between the highest point on the wafer and the lowest point on the wafer is almost 2,000 .ANG. yielding a standard deviation of 472 .ANG. or 6.26%. The curve shown in FIG. 1D is plotted with the removal rates in the vertical axis and the distance from the center of the wafer in the horizontal axis. It is seen that the removal rates at the edges of the wafer are substantially higher than the removal rate at or near the center of the wafer. The thickness uniformity on the resulting wafer after the CMP process is therefore very poor.
A more detailed cross-sectional view of the improved CMP 20 is shown in FIG. 4. The CMP head 20 further includes a wafer mounting plate 30, a bumper ring 32, an inner tube 34 for supplying the pneumatic force 18 (shown in FIG. 1C) and a base plate 36. The bumper ring 32 is utilized between the wafer 10 and the mounting plate 30 for preventing edge defect by raising the edges of wafer 10 when pressed down onto a polishing pad (not shown). Without the use of the bumper ring 32, the edge portion of the wafer 10 is not polished to the same degree as the center portion of the wafer 10 and therefore, the bumper ring 32 compensates for the poor polishing along the edges of wafer 10 by providing a support behind the wafer. Both the bumper ring 32 and the wafer mounting plate 30 are normally fabricated of a rigid material such as plastic or ceramic. The wafer mounting plate 30 is further provided with a plurality of through holes 40, as shown in FIG. 2A.
FIGS. 2A and 2B illustrate a plane view and a side view, respectively of the wafer mounting plate 30 shown in FIG. 4. The plurality of through holes 40, or apertures, are provided for fluid communication between an upper surface 38 and a lower surface 42 of the wafer mounting plate 30 which enables a vacuum to be exerted on the wafer 10 when positioned thereunder. It should be noted that the flexible membrane member 16, shown in FIG. 4, is not shown in FIGS. 2A and 2B for simplicity reasons. The flexible membrane member 16 may be fabricated of a breathable material, or a material that is perforated such that vacuum can be pulled on the membrane member for acting on the wafer surface. The flexible member 16 may be advantageously fabricated of an elastomeric material, such as a silicon rubber, a polyurethane rubber or any other high temperature and chemical resistant rubber that does not cause particle contamination.
In the configuration shown in FIGS. 2A, 2B and 4, the wafer mounting plate 30 when used to mount wafer 10 frequently encounters a wafer breakage problem since both the mounting plate 30 and the bumper ring 32 are fabricated of rigid materials and thus causing a stress concentration on the wafer it carries. The breakage of the wafer frequently occurs during a wafer loading or unloading from a load cup, or during wafer chuck or dechuck from a polishing pad. When the breakage occurs, the throughput of the fabrication process is severely affected.
In modern semiconductor devices wherein multi-level interconnects are frequently formed by a plug process, the successful implementation of a CMP process on either tungsten or copper becomes a critical processing step. Frequent wafer breakage problem due to stress concentration imposed by the wafer mounting plate and the bumper ring is therefore unacceptable. It has been found that stress concentration is the single most important reason that causes wafer breakage during loading or unloading of wafer from a carrier head.
It is therefore an object of the present invention to provide an apparatus for mounting a wafer in a chemical mechanical polishing apparatus that does not have the drawbacks or shortcomings of the conventional mounting apparatus.
It is another object of the present invention to provide an apparatus for mounting a wafer in a chemical mechanical polishing process that does not cause wafer breakage problem during loading or unloading of wafers.
It is a further object of the present invention to provide a wafer mounting plate for use in a chemical mechanical polishing apparatus which has a concave surface for contacting a wafer that does not cause stress concentration on the wafer surface.
It is another further object of the present invention to provide an apparatus for mounting a wafer in a chemical mechanical polishing apparatus which is a single piece mounting plate instead of a two-piece mounting plate/bumper ring.
It is yet another object of the present invention to provide a wafer mounting plate for use in a chemical mechanical polishing apparatus which has a concave surface for eliminating stress concentration on a wafer when the wafer is picked up and a plurality of apertures through which a vacuum may be exerted on the wafer.
It is still another further object of the present invention to provide a method for holding a wafer during a polishing process without creating stress concentration on the wafer by using a wafer mounting plate that has a concave surface for contacting the wafer without causing stress concentration.
It is yet another further object of the present invention to provide a method for holding a wafer during a chemical mechanical polishing process that is equipped with a concave surface for contacting a wafer without causing stress concentration and a plurality of apertures through which a vacuum may be exerted on the wafer.